The subject matter disclosed herein relates to magnetic resonance imaging (MRI), and, more particularly, to systems and methods for shim current calculation for shimming a magnet.
In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within a subject of interest, such as a patient. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium directions, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, RF fields are emitted by the spinning, precessing nuclei and are detected by either the same transmitting RF coil, or by one or more separate coils. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
It is typically desirable for the magnetic fields produced by the primary magnet and used to generate the images in such MRI systems to be highly uniform, static magnetic fields. However, the magnetic field produced by the primary magnet within an MRI imager is typically inhomogeneous, for example, due to factors such as the presence of materials (e.g., iron) in the environment that are susceptible to magnetization in the presence of the primary magnet. Further, when the subject of interest is placed within the MRI imager for examination, additional inhomogeneities may be introduced, thus further distorting the desired uniformity of the magnetic field. Accordingly, in many instances, it may be desirable to shim the primary magnet to adjust the homogeneity of the magnetic field in an attempt to correct for the introduced inhomogeneities. However, current techniques employing such methods are often inadequate, or are subject to further improvement. For example, many current shimming techniques require substantial amounts of time to acquire required data and perform the calculations necessary to determine the amount of necessary shimming, thus reducing productivity. Accordingly, it is now recognized that a need exists for improved shimming systems and methods in magnetic resonance imaging that address one or more of the drawbacks associated with current methods.